Back-to-Africa introductions of Mycobacterium tuberculosis as the main cause of tuberculosis in Dar es Salaam, Tanzania

In settings with high tuberculosis (TB) endemicity, distinct genotypes of the Mycobacterium tuberculosis complex (MTBC) often differ in prevalence. However, the factors leading to these differences remain poorly understood. Here we studied the MTBC population in Dar es Salaam, Tanzania over a six-year period, using 1,082 unique patient-derived MTBC whole-genome sequences (WGS) and associated clinical data. We show that the TB epidemic in Dar es Salaam is dominated by multiple MTBC genotypes introduced to Tanzania from different parts of the world during the last 300 years. The most common MTBC genotypes deriving from these introductions exhibited differences in transmission rates and in the duration of the infectious period, but little differences in overall fitness, as measured by the effective reproductive number. Moreover, measures of disease severity and bacterial load indicated no differences in virulence between these genotypes during active TB. Instead, the combination of an early introduction and a high transmission rate accounted for the high prevalence of L3.1.1, the most dominant MTBC genotype in this setting. Yet, a longer co-existence with the host population did not always result in a higher transmission rate, suggesting that distinct life-history traits have evolved in the different MTBC genotypes. Taken together, our results point to bacterial factors as important determinants of the TB epidemic in Dar es Salaam. Author summaryTuberculosis (TB) is among the deadliest human infectious diseases caused by one single agent, Mycobacterium tuberculosis (Mtb). The origins of Mtb have been traced to East Africa millennia ago, where it likely became adapted to infect and transmit in humans. Here, we show that in Dar es Salaam, Tanzania, an East African setting with a high burden of TB, infections are caused by distinct Mtb genotypes introduced in recent evolutionary times from different parts of the world. These genotypes differed in traits important to Mtb transmission; while some Mtb genotypes transmitted more efficiently during a given period of time, patients infected by other genotypes remained infectious for longer. These traits evolved independently in the different Mtb genotypes and could not be explained by the time of co-existence between the host population and the pathogen. This suggests that bacterial factors are important determinants of the TB epidemic. More generally, we demonstrate that distinct pathogenic life history characteristics can co-exist in one host population.